The present invention relates to methods of measuring the calcium plus magnesium hardness of liquids; and, more particularly, to a method of preparing liquid samples for such measurements.
Titration with (ethylenedinitrilo)tetra-acetic acid (EDTA) is a standard method for measuring calcium plus magnesium hardness of water. (Standard Methods For The Examination of Water and Wastewater, pp. 133-37 (11th ed. 1960). In such methods, the calcium plus magnesium hardness is commonly measured in terms of the equivalent concentration of calcium carbonate (mg/l CaCO.sub.3).
It is well known that certain metal ions, such as the copper (II) and iron (III) ions interfere in the standard calcium plus magnesium hardness titration procedures. The interference may arise because the metal ions are co-titrated or because they poison the indicator. The mechanism of poisoning is not fully understood. Poisoning may involve formation of metal complexes with the indicator dyes that are relatively stable compared to the chelates the metal ions form with EDTA, resulting in blocking of the indicator and fading or indistinct endpoints. Poisoning may also be caused by an irreversible chemical reaction of the dye which destroys its utility as an indicator. Interference, by whatever mechanism, is a problem in photometric methods as well as in titration methods.
Various reagents have gained acceptance as masking agents or inhibitors for use in reducing the interference of metal ions. For instance, sodium cyanide and potassium cyanide are recognized masking agents, which owe their effectiveness to the fact that cyanide forms very stable complexes with many metal ions. Another recognized reagent is sodium sulfite, which precipitates many metal ions. The usefulness of the cyanide and sulfite reagents is offset, however, because they are highly poisonous. Other masking agents such as cupferron for copper (II) ions and salicylaldoxine for iron (III) ions produce colored species that interfere in photometric methods.
(Cyclohexylenedinitrilo)tetra-acetic acid (CDTA) is used as a masking agent in the method for determining total residual metal ion hardness of boiler water that is described in U.S. Pat. No. 3,895,913. Heavy metal ion interference with the color indicator Calmagite (1-(hydroxy-4-methyl-2-phenylzao)-2 naptho-4-sulfonic acid) is avoided in this method by adding to the sample of boiler water being tested a solution that contains both CDTA and an equimolar portion of a calcium or magnesium salt. CDTA forms stable chelates with magnesium and calcium, and is therefore present as the calcium or magnesium chelate in the solution that is added to the sample. Copper (II), iron (III), and other metal ions that would interfere with the Calmagite dye form chelates with CDTA that are more stable than the magnesium-CDTA or calcium-CDTA chelates. Stability Constants of Metal-Ion Complexes, Supplement No. 1, pp. 724-28 (The Chemical Society, 1973). Therefore, in the method described in U.S. Pat. No. 3,895,913, the magnesium or calcium-CDTA chelate functions as an exchange reagent. The CDTA sequesters heavy metal ions and an equivalent number of magnesium or calcium hardness ions are released. This prevents heavy metal interference with the indicator, but since the hardness measurement is based on the number of magnesium and calcium ions present, this method effectively includes these additional heavy metal ions in the hardness value.